Low pressure electrostatic precipitator

ABSTRACT

A METHOD OF OPERATING AN ELECTROSTATIC PRECIPITATOR TO CLEAN LOW DENSITY GAS STREAMS. THE SPARKING POTENTIAL OF THE GAS STREAM TO BE CLEANED IS DETERMINED, A CORONA QUENCHING AEROSOL IS ADDED TO INCREASE THE SPARKING POTEN-   TIAL OF THE STREAM AS IT IS PHASED THROUGH A PRECIPITATOR FOR ELECTROSTATIC CLEANING.

March 9, 1971 A. B. WALKER LOW PRESSURE ELECTROSTATIC PRECIPITATOR FiledMay 6, 1968 GOL. 4mm 2m 0 JOmOEm .LJ /VW maaano vwoaoo INVENTOR ALANB.WALKER AT TORN EY-i United States Patent Oifice 3,568,404 PatentedMar. 9, 1971 ABSTRACT OF THE DISCLOSURE A method of operating anelectrostatic precipitator to clean low density gas streams. Thesparking potential of the gas stream to be cleaned is determined, acorona quenching aerosol is added to increase the sparking potential ofthe stream as it is passed through a precipitator for electrostaticcleaning.

BACKGROUND OF THE INVENTION This invention relates generally to cleaningof low density gases resulting from low pressures and/or hightemperature operation and more particularly to a new and improved methodof cleaning such gases by electrostatic precipitation.

Electrostatic precipitators have found wide use in industry for cleaninggases, particularly in operations where a minimum pressure drop in thegas stream is desired. Such operations have, however, been heretoforelimited to gases at substantially atmospheric density due to reducedefficiency of the electrostatic process at low gas densities.

It is important, in particular situations, to clean low density gaseswith a minimum pressure drop through the cleaning operation. In suchgases, the electrical impedance of the gas is low and the currentdensity at which sparking in an electrostatic precipitator will occur isencountered at a relatively low voltage. The maximum field strength forthe precipitator, upon which the precipitation rate and efiiciency arecritically dependent, is therefore limited, and precipitation cleaningof such gases has, heretofore, been considered impractical.

SUMMARY OF THE INVENTION This invention overcomes the disadvantages ofthe prior art by providing means for operating an electrostaticprecipitator on low density, aerosol bearing gases in such a manner asto provide efiicient, effective removal of the aerosol therefrom. Thisis accomplished by increasing the aerosol space charge in the gas to becleaned by adding a secondary or corona quenching aerosol thus providinggreater electrical impedance in the gases and thereby allowingimposition of greater precipitator operating field strengths atequivalent current densities.

The invention also provides a method for elevating the sparking voltagein an electrostatic precipitator operating on low density gases byincreasing the electrical impedance of the gas to be cleaned through anincrease in the concentration of aerosol present in the gas beingtreated.

In a preferred embodiment, the invention provides means for operating anelectrical precipitator to remove solids or liquid aerosols from a gasstream flowing at density ratios relative to air at NTP not greater thanabout 0.5. The sparking potential of the gas stream to be cleaned isdetermined and a corona quenching or secondary aerosol is added to thegas stream, prior to precipitation, in an amount to increase thesparking potential of the gas stream at least about 100% and allowhigher operating voltages and precipitation rates.

These and other objects and advantages of the invention will becomebetter understood to those skilled in the art by reference to thefollowing detailed description when viewed in light of the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of arecipitator and aerosol generator apparatus adapted to clean low densitygases in accordance with the invention; and

FIG. 2 shows current-voltage curves for typical electrode geometry attwo air densities relative to standard conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, a feed duct 10conducts a stream of gas to be cleaned to a mixing zone 12 comprising aventuri 14 having a nozzle 16 concentrically disposed therein. Thenozzle 16 is communicative, through a tube 18 with an aerosol generator20 of any type known in the art. The mixed gases flow from the mixingzone 12, through a duct 22 and expansion chamber 24 into an electricalprecipitator 26 for operation thereon by the precipitator as is known inthe art.. The treated clean gases are then exhausted from theprecipitator through an exhaust duct 28.

The precipitator 26 may be any of the conventional electrostatic typesknown in the art. Such precipitators utilize a corona discharge tocharge aerosol present in the gas stream flowing therethrough.Subsequently, the charged aerosol is precipitated from the stream as itpasses through or between oppositely charged collection tubes, plates,or other surfaces.

As was indicated above, electrostatic precipitator elfectiveness dependsupon the strength of the field in the precipitator. The maximum fieldstrength possible is limited to the sparking voltage which, in turn, isa function of the electrical impedance of the gas. The electricalimpedance of a given gas is determined primarily by its density up totemperatures of about 1500 F., with impedance decreasing with decreasingdensity. Low density gases thus have low electrical impedance. Withgases substantially below atmospheric density, field strength has beenlimited to such low levels that it has been heretofore impractical touse the electrostatic precipitator for cleaning such gases.

In this invention, the electrical impedance or initial aerosol loadingof the incoming, low density gases is first measured by any suitablemeans known in the art prior to entrance thereof into the mixing zone12. A secondary aerosol is then added through the nozzle 16 in quantityand size sufiicient to increase the sparking potential of the gases to asuitable level.

The following illustrative example will demonstrate, byorder-of-magnitude calculation, the method of this patent. Let i(statamp/cm.) be the current per unit length in a wiretube precipitatorbefore the introduction of the secondary aerosol, and i, be the reducedcurrent afterward. If i i the greater part of the reduced current iscarried by aerosol particles (rather than as free ions), and it isapproximately true that the particle concentration C (particles/cm?) isgiven by where p (statcoul./cm. is the space-charge density and q(statcouL/particle) is the charge acquired by a single particle. Theinitial space charge density is =12 P ICE where i (statamp/cm. is theinitial current density, k ((cm/sec.)/(statvolt/cm.)) the ion mobility,and E (statvolt/cm.) the electric-field intensity. Particles charging bythe ion-bombardment process obtain a charge where p (dimensionless) is afunction of the particles dielectric constant (1 p 3) and a (cm.) is theparticle radius. The field is given approximately by measured. Using theforegoing expressions, Equation 1 may be rewritten or in terms of theparticulate mass per unit volume of gas C where pp (gm./cm. is the trueparticle density.

FIG. 2 shows mesaured current-voltage curves for a typical electrodegeometry (0.094-in. diameter wire in a 6.06 diameter tube) at two airdensities 6 relative to standard conditions, namely 6:098 and 0.46. At avoltage of 40 kv., the corona current in the former instance is 0.5rna./ft., and in the latter 3.0 ma./ft. Points A and A in FIG. 2illustrate the points where sparking occurred. We seek to determine theparticle concentration and size of the secondary aerosol which at 6:046will reduce the initial current of 3 ma./ft. to 0.5 ma./ft. Since, to afirst approximation sparkover at different gas densities (for a givengas and geometry) occurs at the same current density, and since thepresence of heavy space charge effectively raises the corona-startingvoltage, the secondary aerosol will serve, very roughly, to shift thecurrentvoltage curve from that given for 5:0.46 to that for 8:0.98. Weintroduce the following quantities into Equation 7:

i ==3 ma./ft.=3 10 statamp/cm. i =0.5 ma./ft.=5 10 statamp/cm. a=1micron= om.

r=1.5 in. (average value of radium vector)=3.8 cm.

whence C=2.1 grains/ft. of 2-micron particles For particles less than amicron in size, Equation 3, whence 'Equation 7, are no longerapplicable. Instead we have from Equation 1, 2, 4 and 5 2a d I s l i 04.37x10 (1 3m 2k (8) The charge q acquired by, say, a 0.2-micronparticle by ion diffusion may be estimated at about 10 electroniccharges or 4.8 10 statcoul. Taking k=1400 (cm./ sec.)/(statvolt/cm.) anda=0.1 rnicron=10 0111., we have C=0.28 grain/ft. of 0.2-rnicronparticles With the above parameters, the maximum corona voltage was onthe order of 10 times greater than that possible without aerosolaugmentation of the gas stream.

The secondary aerosol may be of any type suitable for the purposes,however, an aerosol selected from the group consisting of metallic oxidefumes or a fine liquid fume such as oil, water vapor, glycol and thelike in wet precipitator operations has been found to be preferable. Inhigh temperature conditions the dry aerosol is preferred. Such aerosolis ideally of submicron size formed by condensation of the aerosolmaterial from its vapor state.

In general the size of the particles added to the gas stream should notbe greater than about .25 micron and preferably in the order of about .1micron or smaller as shown by Equation 7.

What has been set forth above is intended as exemplary of a teaching inaccordance with the invention to enable those skilled in the art in thepractice thereof. It should therefore be understood that, within thescope of the appended claims, the invention may be practiced other thanas specifically described.

\It Will be appreciated that suitable current or voltage, or sparksensing means may be utilized as the control means for the aerosolgenerator to insure optimum operation of the precipitator.

I claim:

1. A method of operating an electrostatic precipitator to remove initialaerosol from a gas stream wherein the density of the gas stream to becleaned relative to air at NTP is not greater than about 0.5 comprisingthe steps of:

determining the sparking potential of the gas stream to be cleaned;

adding a corona quenching aerosol of a size not greater than about .25micron to the gas stream to thereby increase the sparking potential ofthe gas stream; and

electrostatically precipitating said aerosols from said gas stream.

2. The method defined in claim 1 wherein the corona quenching aerosol isproduced from a fine liquid fume selected from the group consisting ofoil, a glycol, and water.

3. The method defined in claim 1 wherein the corona quenching aerosol isa fume produced from a metallic oxide.

References Cited UNITED STATES PATENTS 1,022,012 4/1912 Whitney 3172621,331,225 2/1920 Wolcott 555 1,441,713 1/1923 Prosser 555 1,883,37310/1932 Hedberg 55-5 909,825 5/1933 Hahn et al 55106X 2,356,717 8/1944Williams 23-1 2,726,730 12/1955 MacKenzie 55101X 2,746,563 5/1956 Harlow23175X 2,841,242 7/1958 Hall 23174X 2,864,456 12/1958 Hall et al. 55106X 1,883,372 10/1932 Hedberg 555 2,935,375 5/1960 Boucher 232 3,372,5283/1968 Hoff 55106 3,395,193 7/1968 Bruce et al. 26 0679 FOREIGN PATENTS559,532 2/1944 Great Britain 55-5 932,895 7/1963 Great Britain 55134DENNIS E. TALBERT, JR., Primary Examiner US. Cl. X.R.

